Polypyrrolidone, formed by the polymerization of 2-pyrrolidone, commonly termed Nylon-4 is used for the formation of films and fibers of high strength. Methods for the preparation of polypyrrolidone are described in U.S. Pat. No. 2,638,463 of May 12, 1953.
It has also been suggested in U.S. Pat. No. 2,711,398 of June 21, 1955 to form fibers and films from anhydrous solutions of polypyrrolidone and formic acid. The patent specifies that the utilization of anhydrous formic acid, i.e., 98-100%, is critical to the practice of the invention.
U.S. Pat. No. 2,889,611 to Bedell of June 9, 1959 discloses the treatment of various Nylon polymer filaments with strong solutions of formic acid under such conditions that the portion of Nylon dissolved in the formic acid solution is re-deposited or re-coated upon the external surface of the Nylon filament, which treatment increases the flexibility of the filament without decreasing the tensile strength thereof.
U.S. Pat. No. 3,060,141 discloses a method for preparing shaped articles from aqueous solutions of formic acid. The patent discloses the utilization of aqueous formic acid solutions containing from 10-50% by weight of water.